In a mobile (cellular) communications network, user devices (also known as User Equipment (UE), for example mobile telephones) communicate with remote servers or with other user devices via base stations. In their communication with each other, user devices and base stations use licensed radio frequencies, which are typically divided into frequency bands and/or time blocks.
In order to be able to communicate via the base stations, user devices need to monitor control channels operated by the base stations. One of these control channels, the so-called Physical Downlink Control Channel (PDCCH) carries the scheduling assignments and other control information. The PDCCH serves a variety of purposes. Primarily, it is used to convey the scheduling decisions to individual user devices, i.e. scheduling assignments for uplink and downlink communication.
The information carried on the PDCCH is referred to as downlink control information (DCI). Physical control channels, such as the PDCCH, are transmitted on an aggregation of one or several consecutive Control Channel Elements (CCEs), where a control channel element corresponds to nine Resource Element Groups (REGs). Each REG has four Resource Elements (REs).
Recent developments in telecommunications have seen a large increase in the use of machine-type communications (MTC) user devices which are networked devices arranged to communicate and perform actions without human assistance. Examples of such devices include smart meters, which can be configured to perform measurements and relay these measurements to other devices via a telecommunication network. Machine-type communications are also known as machine 2 machine (M2M) communications. It is envisaged that MTC user devices will play an important role in the implementation of the concept of the “internet of things”. It is common for MTC devices, such as smart meters or domestic appliances, to remain in a fixed location or exhibit low mobility. Such devices may also be deployed deep inside buildings where network coverage is low. For example, some MTC user devices may be installed in the basement of a residential building or in a location shielded by foil-backed insulation or metallised windows. These MTC devices will experience greater penetration losses on the air interface than normal user devices.
The lack of network coverage, in combination with the often limited functionality of MTC user devices, can result in such MTC user devices having a low data rate and therefore there is a risk of some messages or channels, such as the PDCCH, not being received by an MTC user device. In order to mitigate this risk, it is desirable to increase the coverage of the PDCCH (and/or, where applicable, the evolved physical downlink control channel, EPDCCH).
One approach proposed for the enhancement of coverage is the repetition of (E)PDCCH across multiple subframes. However, enhancing coverage presents challenges relating to how carrier frequencies should be aggregated, how and where to signal a control channel in radio frames, and how to ensure that user equipment, including MTC and legacy devices, can efficiently locate and interpret the control signalling.